Directly painted article fabricated from a thermoplastic polyolefin and ethylene polymers blend

ABSTRACT

A polyolefin-EPDM or EPM thermoplastic blend in which the bonded relation of applied organic coatings is improved by the inclusion of an oxidized or carboxylated polyolefin.

This is a divisional of co-pending application Ser. No. 07/319,954 filedon Mar. 7, 1989 and issued on Feb. 5, 1991 as U.S. Pat. No. 4,990,568which was a continuation of co-pending application Ser. No. 07/008,633filed on Jan. 29, 1987, now abandoned.

INTRODUCTION

This invention is addressed to the problem of adherence of coatingsapplied to surfaces of articles formulated of such thermoplasticmaterials as are referred to in the trade as thermoplastic elastomers(TPE) or thermoplastic olefinic elastomers (TPO), such asEPDM-polypropylene compositions.

BACKGROUND OF THE INVENTION

Many automotive and other articles are molded or otherwise fabricated ofthermoplastic compositions which embody about equal parts by weight of(1) an interpolymer of ethylene, one or more C₃ -C₁₆ mono-olefins,preferably propylene and a polyene, preferably5-ethylidene-2-norbornene, generally referred to in the trade as an EPDMand (2) a thermoplastic polyolefin resin, preferably polypropylene. Oneof the problems faced by articles molded or otherwise fabricated of suchTPO compositions, is the difficulty of achieving strong adherence ofcoatings applied to the surfaces of such articles, as in the colorcoating of such articles, especially with a polyurethane based coatingcomposition.

To the present, in order to provide for a desirable adhesion of apaintable coating, it has been the practice to first apply an adhesionpromoter, followed by the application of a prime coat, and then theapplication of the final paintable color coat. The ability to eliminatethe promoter as a first coating while achieving strong adherence of thepaintable color coating would materially enhance the entire processespecially from the standpoint of material cost, processing time andconservation of space, equipment, and labor.

OBJECT OF THE INVENTION

Thus it is an object of this invention to provide a TPO composition ofthe type described which can be directly paintable (DP-TPO) to providestrong adherence of applied coatings without the need of a firstadhesion promoter coating and in which such directly coatable orotherwise paintable article can be achieved by modification of thecomposition of which the article is formed but without undesirableeffect on the physical or chemical properties of the article.

BRIEF DESCRIPTION OF THE INVENTION

It has been found that the objectives described can be achieved when theTPO composition is formulated to contain 2-20 percent and preferably4-15 percent by weight of an oxidized or carboxylated polyolefin,preferably an oxidized polyethylene.

It is preferable to substitute the desired amount of oxidizedpolyethylene for polypropylene in the composition, but the desiredresults will be experienced when the carboxylated polyethylene oroxidized polyethylene is present in the described amounts whethersubstituted for the polypropylene or the EPDM or both in the TPOcomposition.

DETAILED DESCRIPTION OF THE INVENTION

As previously pointed out, one of the principal components of the TPOcomposition of which the articles are molded or otherwise fabricatedcomprises a thermoplastic polyolefin resin which is a homopolymer orcopolymer of one or more C₂ -C₁₀ mono-olefins such as polyethylene andpreferably polypropylene, which are polymers well known to the trade andreadily commercially available, or mixtures thereof. Generally suchthermoplastic polyolefin resin is present in the TPO composition in anamount of up to 75 percent by weight of rubber plus resin, andpreferably in an amount within the range of 30-70 percent by weight ofthe composition. When the resin is present in an amount greater than 70percent by weight of the composition, the composition behaves more likean impact modified thermoplastic than a TPO. It will be apparent tothose skilled in the art that such impact modified thermoplasticscontaining the oxidized polyolefin as an additive would also exhibitimproved paint adhesion characteristics. Thus the useful range ofcompositions includes levels of resin up to 90 parts by weight per 100parts by weight of the total blend.

The other principal component of the TPO composition is a copolymer ofethylene and a C₃ -C₁₆ mono-olefin, preferably an ethylene-propylenecopolymer rubber (EPM) and preferably an interpolymer of (1) ethylene,(2) a C₃ -C₁₆ mono-olefin, preferably propylene and (3) a polyene (EPDM)such as 5-ethylidene-2-norbornene. Such other component is present inthe TPO composition an amount within the range of 30-70 percent byweight and preferably within the range of 40-60 percent by weight.

As the EPM rubber, use can be made of an ethylene-mono-olefin andpreferably an ethylene-propylene copolymer rubber formed bycopolymerization in solvent solution, in the presence of a Ziegler-typecatalyst, of ethylene and one or more mono-olefins, preferablypropylene, but which may include 1-butene, 1-pentene, or othermono-olefins having 3-12 carbon atoms. The ratio of ethylene topropylene or C₃ -C₁₂ mono-olefin may range from 10-95 moles of ethyleneto 90-5 moles of propylene or other mono-olefins. The preferred range ofethylene to propylene or other mono-olefin is 45-85 moles of ethylene to55-15 moles of propylene or other mono-olefin.

In the preparation of the EPDM interpolymer rubber, the polyene monomercontaining a plurality of carbon-to-carbon double bonds may be selectedfrom those disclosed in the prior art for use as third monomers in thepreparation of ethylene-mono-olefin-polyene terpolymers, including openchain poly-unsaturated hydrocarbons containing 4-20 carbon atoms, suchas 1,4-hexadiene, monocyclic polyenes and polycyclic polyenes. Thepolyunsaturated bridged ring hydrocarbons or halogenated bridged ringhydrocarbons are preferred. Examples of such bridged ring hydrocarbonsinclude the polyunsaturated derivatives of bicyclo(2,2,l)heptane whereinat least one double bond is present in one of the bridged rings, such asdicyclopentadiene, bicyclo-(2,2,1)hepta-2,5-diene, the alkylidenenorbornenes, and especially the 5-alkylidene-2-norbornenes wherein thealkylidene group contains 1-20 carbon atoms and preferably 1-8 carbonatoms, the alkenyl norbornenes, and especially the5-alkenyl-2-norbornenes wherein the alkenyl group contains about 3-20carbon atoms and preferably 3-10 carbon atoms Other bridged ringhydrocarbons include polyunsaturated derivatives of bicyclo(2,2,2)octane as represented by bicyclo(3,2,l) octane, polyunsaturatedderivatives of bicyclo(3,3,l) nonane, and polyunsaturated derivatives ofbicyclo(3,2,2) nonane.

Specific examples of preferred bridged ring compounds include5-methylene-2-norbornene, 5-ethylidene-2-norbornene,5-n-propylidene-2-norbornene, 5-isobutylidene-2-norbornene,5-n-butylidene-2-norbornene, dicyclopentadienes; the methyl butenylnorbornenes such as 5-(2-methyl-2-butenyl)-2-norbornene or5-(3-methyl-2-butenyl)norbornene, and5-(3,5-dimethyl-4-hexenyl)-2-norbornene. The elastomer prepared from5-ethylidene-2-norbornene is much preferred as it has outstandingproperties and produces many unusual and unexpected results.

The EPDM backbone rubber may contain chemically bound therein molarratios of ethylene to propylene or other C₃ -C₁₆ mono-olefin varyingbetween 95:5 to 10:90 ethylene:propylene, and preferably between 85:15to 55:45 ethylene:propylene. The polyene or substituted polyene may bechemically bound therein in an amount of 0.1 to 10 mol percent, andpreferably 0.3 to 1 mol percent. The level of unsaturation in thebackbone rubber may range for 0-15 double bonds per 1,000 carbon atomsin the polymer chain.

The interpolymerization is carried out in the presence of a Zieglercatalyst of the type well known to the prior art. Such Ziegler typecatalysts are disclosed in a large number of patents, such as U.S. Pat.Nos. 2,933,480, 3,093,620, 3,093,621, 3,211,709 and 3,113,115. Examplesof Ziegler catalysts include metal organic coordination catalystsprepared by contacting a compound of a heavy metal of the group IV-a,V-a, VI-a and VII-a of the Mendeleeff periodic system of elements, suchas titanium, vanadium and chromium halides with an organo-metalliccompound of a metal of groups I, II or III of the Mendeleeff periodicsystem which contains at least one carbon-metal bond, such as trialkylaluminum, and alkyl aluminum halides in which the alkyl groups containfrom 1-20 and preferably 1-4 carbon atoms.

The preparation of EPM and EPDM polymers is well known and is fullydescribed in such patents as U.S. Pat. Nos. 2,933,480, 3,093,621,3,211,709, 3,646,168, 3,790,519, 3,884,993, 3,894,999 and 4,059,654,amongst many others.

Oxidized polyethylene wax materials suitable for use in the practice ofthis invention are described in U.S. Pat. No. 3,060,163. According tothis patent normally solid, hard, waxy polymers of ethylene having anaverage molecular weight between about 1,000 and 3,000 are subjected, inthe liquid phase, to the action of an oxygen-containing gas to causereaction of between 2-17 pounds of oxygen per 100 pounds of wax, i.e. toprovide an oxidized polyethylene wax containing at least 1 percent andpreferably 1-8 percent by weight of oxygen, and acid numbers of not morethan about 50, and preferably between 10 and 45.

In the preferred practice of this invention, the oxidized polyethyleneis characterized by having a minimum number average molecular weightabove 1000 and preferably at least about 1200, as determined by hightemperature vapor pressure osmometry, containing between 1-5 percent byweight of total oxygen, and having an acid number of from 10 to about35. The described oxidized polyethylene is obtained by oxidation ofpolyethylene in molten or finely divided solid form, with free oxygencontaining gas, usually air, generally at elevated temperature, untilthe desired oxygen content is obtained. Starting materials for makingthe oxidized polyethylene suitable for use in the practice of thisinvention include low molecular weight, low density or linear lowdensity polyethylene waxes having densities in the range of about 0.91to about 0.96 as, for example, prepared by the process described in U.S.Pat. No. 2,683,141, as well as high density, linear polyethylene as, forexample, prepared in the presence of such well know catalysts as the"Phillips" or "Ziegler" type catalysts, having densities in the rang ofabout 0.93-0.97 or above. The low molecular weight, low densitypolyethylene starting material can be oxidized by contacting in themolten state with a stream of air until the desired oxygen content hasbeen obtained. The high density, linear polyethylene starting materialis usually oxidized by contact, preferably in the finely divided solidstate, with free oxygen-containing gas, usually air, at temperaturesranging from 100° C. up to, but not including, the crystalline meltingpoint of the polyethylene, until the desired oxygen content has beenobtained.

A preferred low molecular weight, low density oxidized polyethylene ismarketed by Allied Chemical Corporation under the designation AC-629.This oxidized polyethylene product has a specific gravity of about0.9±0.05, a number average molecular weight of about 1300, an oxygencontent of between 3 and 5, and an acid number of about 14 to 17.

Carboxylated polyethylenes which may be employed in the practice of thisinvention can be prepared by thermal degradation of a high molecularweight polyethylene in the presence of air or other oxygen-containinggas at an elevated temperature, so as to reduce the molecular weight andintroduce carboxylic acid groups at the points of chain scisson.Polyethylene having a molecular weight from about 100,000 to 1,000,000,and a specific gravity of 0.91 to about 0.965 may advantageously beused. For example, a polyethylene having a specific gravity of 0.94, aparticle size of 100 mesh (150μ) and an original viscosity averagemolecular weight of 2.6×10⁶ was stirred constantly and heated to 260° F.An air stream heated to 230° F. was passed through the particles. After13 hours the molecular weight was reduced to about 500,000 and to 60,000after 16 hours. Ordinarily, carboxylic acid groups are introduced ateach point of chain scisson, such that each molecule will have at leastone carboxylic acid group, and preferably two such groups at lowmolecular weight. An alternative method for the preparation of acarboxylated polyolefin is by reaction with a polycarboxylic acid, suchas maleic, at high temperature. One method of preparation of suchmaterials is described in U.S. Pat. No. 2,766,214.

Having described the basic concepts of this invention and the componentsthereof, description of the invention will now be made by way of thefollowing examples, which are given by way of illustration and not byway of limitation.

The components which are formed into a composition for fabrication intovarious products, as by way of conventional molding procedure, areadmixed for bulk reaction at a temperature above the melting pointtemperature of the thermoplastic resinous component, such as at atemperature above 325° F. up to a temperature of 600° F. and preferablyup to about 500° F., as in hot melt processing equipment, such as aBanbury or extruder, for from 3-5 minutes.

The procedure for applying the color coating to articles fabricated fromthe compositions of this invention was carried out as follows:

1. The surface of the fabricated product was first cleaned with anacidic or alkali wash.

2 A Durethane 400 602-1060 primer was applied in an amount to provide adry film of 1.0±0.2 mils.

3. A second coating of primer was applied after a two minute flash offirst coat under ultra-violet light (UV).

4. The second coating was baked under ultra violet for 15-20 minutes at250° F.

5. A Durethane 700 HSE Topcoat was applied to provide a dry film of 1.8mils.

6. A second Durethane Topcoat was applied after a 2 minute flash underUV.

7. The second topcoat was baked under UV for at least 20 minutes at 250°F.

A conventional test for adhesion was employed for evaluation of the bondbetween the topcoat and the surface of the fabricated article. In theadhesion evaluation, referred to in the trade as the "Crosshatch" test,a razor blade is used to cut 11 vertical lines spaced 1/8 inch apart,followed by cross cutting in the same spaced relation to provide 100squares of 1/8 inch. A pressure sensitive cellophane tape is pressedonto the test area and then it is ripped off. The number of squareswhich remain bonded to the surface identifies the percent crosshatch. Inother words, a value of 100% crosshatch indicates that all of thesquares remain bonded to the surface. A 90% crosshatch indicates that10% of the squares were removed with the tape. A value of 0% crosshatchindicates that all of the squares were removed with the tape and thatnone remained on the surface of the test piece.

EXAMPLE 1

A typical TPO compound, yielding 60,000 psi flexure modulus, andcharacterized by very poor paint adhesion, as exhibited by % crosshatchadhesion, had the following composition:

    ______________________________________                                        EPDM interpolymer having an                                                                         50%                                                     ethylene-propylene ratio of                                                   80/20 and about 5 C═C double                                              bonds per 1000 carbon atoms                                                   (EPsyn ® 5508 of Copolymer                                                Rubber & Chemical Corporation                                                 of Baton Rouge, Lousiana)                                                     12 melt flow polypropylene                                                                          50%                                                     Paint adhesion, % crosshatch                                                                        0                                                       Tensile strength, psi 2750                                                    Elongation, %         775                                                     Flow Rate g/10 min    2.0                                                     (Condition "L")                                                               ______________________________________                                    

EXAMPLE 2

Use is made of the typical TPO compound (same as in example 1) but inwhich 10 parts by weight of the polypropylene has been replaced with 10parts by weight of oxidized polyethylene:

    ______________________________________                                                           By Weight                                                  ______________________________________                                        EPsyn ® 5508     50%                                                      12 melt flow polypropylene                                                                         40%                                                      Allied AC oxidized polyethylene                                                                    10%                                                      Paint adhesion, % crosshatch                                                                       100                                                      Tensile strength, psi                                                                              3000                                                     Elongation, %        1100                                                     Flow Rate g/10 min.  2.8                                                      (condition "L")                                                               ______________________________________                                    

It will be seen from the above, that not only is paint adhesionexcellent, but that tensile strength, elongation and flow rate are alsoimproved.

EXAMPLE 3

This example illustrates a typical TPO compound (same as in example 1)but in which 10 parts by weight of the polypropylene has been replacedwith 10 parts by weight of the polyethylene which has not been oxidized:

    ______________________________________                                                           By Weight                                                  ______________________________________                                        EPsyn ® 5508     50%                                                      12 melt flow polypropylene                                                                         40%                                                      Allied AC-617A non-oxidized                                                                        10%                                                      polyethylene                                                                  Paint adhesion, % crosshatch                                                                       0                                                        Tensile strength, psi                                                                              2200                                                     Elongation, %        650                                                      Flow rate g/10 min.  3.6                                                      (condition "L")                                                               ______________________________________                                    

It will be noted that in the presence of non-oxidized polyethylene theadhesion is poor. Before the discovery of the instant invention thisdeficiency was overcome by the incorporation of an additionalpretreatment step, prior to the application of the topcoat, involvingthe treatment of the object to be coated with an adhesion promoter. Thisstep is now made unnecessary by the practice of the instant invention.

EXAMPLES 4-8

The following examples illustrate the effect of the level of addedoxidized polyethylene on the paint adhesion properites.

    ______________________________________                                        Formulation  Examples                                                         (parts by weight)                                                                          4      5       6     7      8                                    ______________________________________                                        EPsyn ® 5508                                                                           50     50      50    50     50                                   12 MF polypropylene                                                                        48     46      44    42     40                                   N550 Black   .5     .5      .5    .5     .5                                   Oxidized     2      4       6     8      10                                   polyethylene                                                                  Paint adhesion,                                                                            0      10      100   100    100                                  % crosshatch                                                                  ______________________________________                                    

It can be seen from the above examples that cross-hatch adhesionincreases with increasing levels of oxidized polyethylene, reaching themaximum level of 100% adhesion at a level of 6 parts by weight per 100parts by weight of rubber plus resin plus oxidized polyethylene.

EXAMPLES 9-14

The following examples illustrate the effect of the rubber/resin ratioon the paint adhesion properties at a constant level of oxidizedpolyethylene.

    ______________________________________                                        Formulation  Examples                                                         (parts by weight)                                                                          9      10     11   12   13    14                                 ______________________________________                                        EPsyn ® 5508                                                                           0      10     20   30   40    50                                 12 MF polypropylene                                                                        90     80     70   60   50    50                                 N550 Black   .5     .5     .5   .5   .5    .5                                 Oxidized     10     10     10   10   10    10                                 polyethylene                                                                  Paint adhesion,                                                                            0      25     68   92   100   100                                % crosshatch                                                                  ______________________________________                                    

It can be seen from the above examples that cross-hatch adhesionincreases with increasing rubber level, reaching the maximum value of100% at a level of 40 parts by weight of rubber per 100 parts by weightof rubber plus resin plus oxidized polyethylene. This implies thathigher levels of oxidized polyethylene are needed to achieve maximumpaint adhesion as the level of polypropylene in the blend is increased.This is further supported by Example 15 which demonstrates in comparisonwith Example 12 that increased oxidized polyethylene raises thecrosshatch adhesion for a given level of polypropylene.

EXAMPLE 15

    ______________________________________                                        Formulation        Parts by Weight                                            ______________________________________                                        EPsyn ® 5508   25                                                         12 MI Polypropylene                                                                              60                                                         N550 Black         0.5                                                        Oxidized Polyethylene                                                                            15                                                         Paint adhesion, % crosshatch                                                                     100                                                        Tensile strength, psi                                                                            3300                                                       Elongation, %      100                                                        Flow Rate (Condition "L"),                                                                       7.6                                                        g/10 min.                                                                     ______________________________________                                    

It will be apparent that color coating adhesion is increased when theTPO is formulated to contain 4-15 percent by weight oxidizedpolyethylene. Similar improvements in adhesion are realized when theoxidized polyethylene in the foregoing examples are replaced in whole orin part by oxidized or carboxylated polyethylenes such as isotactic,amorphous, high density and low density polyethylenes and other alphaolefin polymers and copolymers.

It is further assumed that the benefit derived from incorporation of theoxidized polyethylene will be realized whether the compositions aresimple blends or contain partially or fully cured rubber as well asoils, fibers, fillers or other additives which are well-known to thoseskilled in the art.

It will be understood that changes may be made in the details offormulation and operation, without departing from the spirit of theinvention, especially as defined by the following claims.

We claim:
 1. A fabricated plastic article having a paint coating on onesurface thereof, said article being fabricated of a compositionconsisting essentially of a substantially uniform admixture of 30-75parts by weight of thermoplastic polypropylene resin, 70-25 parts byweight of a polymer selected from the group consisting of (a) acopolymer of ethylene and a C₃ -C₁₆ mono-olefin, (b) an interpolymer ofethylene, a C₃ -C₁₂ mono-olefin and a polyene, and (c) 4-15 parts byweight of an oxidized or carboxylated polyolefin.
 2. A fabricatedplastic article having a polyurethane paint coating on one surfacethereof, said article being fabricated of a composition consistingessentially of 30-75 parts by weight of a thermoplastic polypropyleneresin, 70-25 parts by weight of a polymer selected from the groupconsisting of (a) a copolymer of ethylene and a C₃ -C₁₂ mono-olefin, (b)an interpolymer of ethylene, a C₃ -C₁₂ mono-olefin and a polyene, and(c) 4-15 parts by weight of an oxidized or carboxylated polyolefin.